SPIM Dynamic Memory

A stack is an easily managed structure. Only a few memory addresses are needed to keep track of it. (Some of these addresses are in the stack pointer and in the frame pointer registers.) As a program executes, the stack grows and shrinks as subroutines are called and exited.

The heap is more like a book shelf. Books are constantly being taken off the shelf from various locations, leaving gaps, and then later returned, filling the gaps.

Here is how a SPIM program requests a block of memory from SPIM's heap. The program uses code 9 for get more space.

li      $a0,xxx   # $a0 contains the number of bytes you need.
                  # This must be a multiple of four.
li      $v0,9     # code 9 == allocate memory
syscall           # call the service.
                  # $v0 <-- the address of the first byte
                  # of the dynamically allocated block

You don't know in advance what range of addresses you will get back for the allocate memory request. The SPIM service returns the first address of a contiguous block with as many bytes as you requested in $a0. (This is similar to a call to malloc() in "C".)